The need for relatively non-aromatic liquid hydrocarbons boiling in the range of about 125.degree. C. to about 625.degree. C. (e.g., aviation turbine fuels, diesel fuels, flashed distillates, solvents, medicinal and other white oils) continues to increase. These products usually, or at least often, contain relatively high percentages of olefins and monocyclic and polycyclic aromatics so that further hydrogenation is desirable to produce higher valued products. The presence of aromatics in diesel fuels may contribute to air pollution. The hydrofinishing of these types of hydrocarbons to reduce unsaturates is becoming ever more important commercially.
Diesel fuels, which typically have a boiling range between about 200.degree. C. to about 360.degree. C., are under ever increasing environmental pressure to have their aromatics levels decreased. The presence of aromatics in diesel fuel can also significantly reduce the quality of the fuel. The accepted measure of diesel fuel quality is the Cetane Number ("C.N.") which is directly related to the types of molecules that are found in the fuel. Running a diesel engine on a lower cetane fuel than it was designed for will upset the combustion process due to a longer ignition delay period. This will result in poorer cold starting performance and higher levels of noise, smoke and legislated emissions. The cetane number of molecules within a class (e.g., normal paraffins) increases with the number of carbon atoms in the molecule. Molecular classes may be ranked in terms of their cetane number for a specific carbon number; normal paraffins have the highest cetane number followed by normal olefins, isoparaffins and lastly by monocyclic naphthenes. Aromatic molecules have the lowest cetane number. For example, naphthalene has a C.N. of about -15; tetrahydronaphthalene (tetralin) has a C.N. of about 7; decahydronaphthalene (decalin) has a C.N. of 48; butylcyclohexane has a C.N. of about 50; and decane has a C.N. of 76. Thus, a process which would saturate the aromatics in diesel fuel and also convert the resulting saturates to acyclics without significant hydrocracking would improve the quality of the diesel fuel with regard to environmental quality as well as efficiency.
The hydrogenation of unsaturated hydrocarbons, particularly aromatic hydrocarbons, to corresponding saturated hydrocarbons using a catalyst comprising platinum and/or palladium supported on alumina is disclosed in U.S. Pat. No. 3,943,053. An improvement over this catalyst can be found in European Patent Application no. 303,332, published Feb. 15, 1989, wherein as a support is utilized a Y-type of zeolite which has been modified to have a higher silica to alumina ratio and a smaller unit cell size than an unmodified zeolite Y. Similar modified Y-type zeolites are known to be used in hydrocracking operations. See, for example, U.S. Pat. No. 4,401,556, issued Aug. 30, 1983. The problem in utilizing the modified Y-type zeolite as in European No. 303,332, is that some hydrocracking activity is present which can hydrocrack the hydrocarbon feedstocks being hydrofinished, resulting in a reduced liquid yield.
A primary object of this invention is to provide a catalyst and process for hydrogenating petroleum fractions containing aromatics as well as olefins while concomitantly minimizing hydrocracking.
Another object of this invention is to provide a catalyst and process for hydrogenating the aromatics and olefins in diesel fuel stocks, converting the saturated aromatics at least in part to acyclic hydrocarbons while concomitantly minimizing hydrocracking thereby increasing the cetane number of the diesel fuel stock.
Another object of this invention is to provide a catalyst for the second stage of a low pressure two stage process wherein a sulfur- and nitrogen-containing diesel boiling range hydrocarbon feedstock is hydrotreated in the first stage, the product of the first stage is separated from the gaseous phase and passed to a second stage wherein it is hydrogenated to reduce the aromatics and olefins, producing additional acyclic hydrocarbons therefrom with minimum hydrocracking, resulting in a product with an increased cetane number.